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PDP-8/L restoration - PSU problems.

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    PDP-8/L restoration - PSU problems.

    I have been looking at restoring one of our PDP-8/L machines. A while ago I brought up the PSU on the bench. Cleaned it and reformed the big capacitors. One of the rectifier bridges had been stolen so it had to be replaced. Then I used a variac and a dummy load. When raising the input volatge with the variac I was surprised to see that as the input voltage was rising across the regulator the output increased above 5V. It actually reached 5.7 volt until it started regulating down to 5V. I was a bit confused since although TTL usually has a absolute maximum around 7V it is probably not so good that there is a spike at turn on. There were some reports that PDP-8/L have had problems with their power supplies in the past.

    I did find found the PSU schematics, but the regulator board was a black box.

    No schematics was found on the 54-08047 so I traced it out myself and simulated in CircuitLab. Directly from reading the schematic though it is evident that the left transistor half of the matched pair need above 5.7V to start conducting at all. It will then bias Q2 which will then eat up the base current from Q3. Is this really a good design?

    There is a crowbar circuit but since it is using the same zener as the regulator itself it will trip above 6.3 V or so (the simulation is consistent with actual measurements). A bit to much for my taste at least. I have seen rumors that there are no crowbar in a PDP-8/L but there is one, indeed.

    I didn't have the proper SPICE parameters but used more standard PNP and NPN models. Anyhow there were the same type of spike shown.

    My plan is to modify the regulator board so that it regulates better. I did a simulation in CircuitLab where I replaced the 5.1V zener with a 3.9V zener. A voltage divider was inserted on the right hand side of the matched pair. From the simulation it looked much better. There are no spike when the input voltage is ramped up and the crowbar now trips at somewhere around 5.3V.

    Next step is to test this in the real circuit to see if works equally well.

    What was the nature of your dummy load?
    Reach me: vcfblackhole _at_ protonmail dot com.


      That looks very similar to the behavior of the H-724 power supply in a PDP-12.

      Member of the Rhode Island Computer Museum


        Originally posted by Chuck(G) View Post
        What was the nature of your dummy load?
        Both the real dummy load and the simulated was a pure resistor.

        I suspect the actual load to be almost only resistive. But note that this is a DC sweep simulation. Not a time or frequency domain simulation.


          Originally posted by m_thompson View Post
          That looks very similar to the behavior of the H-724 power supply in a PDP-12.
          I looked at the schematics for the G824 module. It looks very similar, indeed. I can't see anything of the current limit and fold back that are present in the PDP-8/L though. It was just a quick glance so I can be wrong.


            I would be real careful with playing around with the threshold of the crow bar, know what you’re saying about trying to keep from hitting the five volt bus too hard but I always tend to think sometimes people get too worked up about keeping the five volt bus at exactly five volts. A little bit over five volts makes up for the losses in the distribution and connectors so by the time you get onto a circuit card you may not have five volts any more. When crow bars fire they have a bad habit of sometimes shorting the SCR and have been known to also damage the pass transistors so you don’t want it going off any more then necessary, and in the event of a catastrophic failure in the regulator that usually turns out to be a shorted pass transistor the full unregulated voltage is dumped on the output so you would be looking at something well above five volts.
            Once the crow bar fires its intended to blow F3 and with that it will also remove the gate voltage from the SCR so it should be ok but I have had too many crow bars that once fired the SCR shorts but that’s just me, so it’s just my opinion and not to take away from what you’re doing but I would not mess with the threshold of the crow bar.


              @Qbus: Maybe you are right. In my opinion 6.4V crowbar triggering is far to high (the simulation give the same result as the actual circuit tested with a bench supply). In other supplies I worked with the crowbar trigger around 5.5V which I think is more reasonable. With the design of this PSU were there are a spike in the 5V so that it reaches 5.7V when the supply capacitor is charging at startup they just added a diode forward drop to make it not false trigger. Thus making the crowbar trip around 6.4V...

              By adjusting down to a 3.9V or 4.3V zener I expect crowbar to trigger at 5.4V or 5.8V. I'll give it a try but I keep in mind what you said about short circuit SCRs. In this case they have a 0.1ohm in series with the resistor limiting the current a bit. The 2N4441 is rated at 80A surge current which is exactly what you get with 8V and 0.1ohm...


                Maybe it’s just my paranoia but I have had nothing but misery with SCR being every time they somehow get the gate turned on it manages to short out the device if there is nothing on the cathode. Like a three legged self-destruction device!


                  I noticed that there are at least one more version of the regulator board. I have reverse engineered the one in my 8/L #905 schematics here and pictures in a zip file.

                  I wander if the crowbar is designed for tripping at 6,3V to 6,5V to avoid tripping if the 5V rail reaches 5,7V at startup. Looking at the schematics it looks right that is should trip at this voltage.


                    The absolute max supply voltage for the Vcc bus on TTL is 7.0 Volts. It is recommended that it operated between 4.75 and 5.25 Volts.


                      Originally posted by Qbus View Post
                      Maybe it’s just my paranoia but I have had nothing but misery with SCR being every time they somehow get the gate turned on it manages to short out the device if there is nothing on the cathode. Like a three legged self-destruction device!
                      I happily replace SCRs every time the crowbar trips if it saves tens or hundreds of TTLs or other components.


                        Originally posted by anders_bzn View Post
                        I noticed that there are at least one more version of the regulator board. I have reverse engineered the one in my 8/L #905
                        very strange. not a DEC PCB part number and almost no US manufactured components.


                          Originally posted by dave_m View Post
                          The absolute max supply voltage for the Vcc bus on TTL is 7.0 Volts. It is recommended that it operated between 4.75 and 5.25 Volts.
                          I am aware of the abs max rating. My thinking is that it might be a bad idea to expose 50 year old TTL chips to voltages at 5.7V at above which is what the PSU will output momentarily at startup. Or maybe it is a quick way to weed out marginal chips?


                            I recall that a friend who worked as a DEC CE used to grumble about "transistor-protected fuses".
                            Reach me: vcfblackhole _at_ protonmail dot com.


                              Question: what’s more stressful? Operating vintage electronics at lower then originally rated voltages, at rated voltage or slightly higher voltages? On very old radios with tubes I tend to run them around 10% below the original line voltage keeping in mind that the modern line voltage is slightly higher then what was around at the time of their manufacture and want to try to extend tube life. Others run sets at way reduced voltages to keep from stressing the sets. But a vintage computer is something from maybe the late sixties or early seventies when higher line voltages were common so I run all the vintage computer stuff at regular line voltage with the idea of reduced voltage may produce more stress on power supplies then the design voltage. Something about reduced voltage will require more current to accomplish the same power and its current that’s where the real work occurs in both power supplies and distribution. It may be antidotal but I have noticed that most damage occurs when turning on systems and would speculate that perhaps this occurs because of reduced voltage and increased current for a short period of time and perhaps that’s what pushes devices into failure? Or maybe it’s just the insulation of the gates that fail due to the application of voltage at startup? The real question is what pushes gates into failure upon startup?
                              Attached is a picture of my 1938 Navy HRO receiver, at almost eighty years old it’s still working great. My two PDP-11 systems are half that age and would like to think that they may be around just as long.